3. THE DISCOVERY OF QUASARS

Minkowski's studies of radio galaxies culminated with
identification of 3C 295 with a member of a cluster
of galaxies at the unprecedented redshift of 0.46
(Minkowski 1960).
Allan Sandage of the Mt. Wilson
and Palomar Observatories and Maarten Schmidt of
the California Institute of Technology (Caltech) then took
up the quest for optical identifications and redshifts
of radio galaxies. Both worked with Thomas A. Matthews, who obtained
accurate radio positions with the new interferometer at the
Owens Valley Radio Observatory operated by Caltech.
In 1960, Sandage obtained a photograph of 3C 48 showing a 16m stellar object with
a faint nebulosity. The spectrum of
the object showed broad emission lines at unfamiliar wavelengths,
and photometry showed the object to be variable
and to have an excess of ultraviolet emission compared with
normal stars. Several other apparently star-like images
coincident with radio sources were found to show strange,
broad emission lines. Such objects
came to be known as quasi-stellar radio sources (QSRS),
quasi-stellar sources (QSS), or quasars. Sandage reported
the work on 3C 48 in an unscheduled paper in the December, 1960,
meeting of the AAS (summarized by the editors of Sky and Telescope
[Matthews et al. 1961]).
There was a "remote possibility that it may be a distant galaxy of stars"
but "general agreement" that it was "a relatively nearby star
with most peculiar properties."

The breakthrough came on February 5, 1963,
as Schmidt was pondering the spectrum of the quasar 3C 273. An accurate position had been obtained
in August, 1962 by
Hazard, Mackey, and
Shimmins (1963),
who used the 210 foot antenna at the Parkes station in Australia to
observe a lunar occultation of 3C 273. From the precise time
and manner in which the source disappeared and reappeared,
they determined that the source had two components.
3C 273A had a fairly typical class II radio spectrum,
F ~
-0.9; and it was
separated by 20 seconds of arc from
component `B', which had a size less than 0.5 arcsec and
a "most unusual" spectrum,
f ~
0.0.
Radio positions B and A, respectively,
coincided with those of a 13m star like
object and with a faint wisp or jet pointing away from
the star. At first suspecting
the stellar object to be a foreground star, Schmidt
obtained spectra of it at the 200-inch telescope in late December, 1962.
The spectrum showed broad emission lines at unfamiliar wavelengths,
different from those of 3C 48. Clearly, the object was no ordinary star.
Schmidt noticed that four emission lines
in the optical spectrum showed a pattern of decreasing
strength and spacing toward the blue, reminiscent of the
Balmer series of hydrogen. He found that the four lines
agreed with the expected wavelengths of
H,
H,
H, and
H with a redshift of
z = 0.16. This redshift
in turn allowed him to identify a line in the ultraviolet part
of the spectrum with Mg II
2798.
Schmidt consulted with his colleagues, Jesse L. Greenstein and
J. B. Oke. Oke had obtained
photoelectric spectrophotometry of 3C 273 at the 100-inch telescope,
which revealed an emission-line in the infrared at
7600. With
the proposed redshift, this feature
agreed with the expected wavelength of
H.
Greenstein's spectrum of 3C 48 with a redshift of z = 0.37,
supported by the presence of Mg II in both objects.
The riddle of the spectrum of quasars was solved.

These results were published in Nature six weeks later
in adjoining papers
by Hazard et al. (1963);
Schmidt (1963);
Oke (1963);
and Greenstein and
Matthews (1963).
The objects might be galactic stars with a very
high density, giving a large
gravitational redshift. However, this explanation was
difficult to reconcile with the widths of the
emission lines and the presence of forbidden
lines. The "most direct and least objectionable"
explanation was that the objects were extragalactic,
with redshifts reflecting the Hubble expansion. The redshifts
were large but not unprecedented; that of 3C 48 was second only to that
of 3C 295. The radio
luminosities of the two quasars were comparable with those
of Cyg A and 3C 295. However, the optical luminosities
were staggering, "10 - 30 times brighter than the brightest
giant ellipticals"; and the radio surface brightness
was larger than for the radio galaxies. The redshift of 3C 273
implied a velocity of 47,400 km s-1 and a distance of
about 500 Mpc (for H0 100 km s-1
Mpc-1). The nuclear
region would then be less than 1 kpc in diameter. The jet would be
about 50 kpc away, implying a timescale greater than 105 years
and a total energy radiated of at least 1059 ergs.

Before the redshift of 3C 273 was announced,
Matthews and Sandage
(1963)
had submitted a paper identifying 3C 48,
3C 196 and 3C 286 with stellar optical objects.
They explored the popular notion that these
objects were some kind of Galactic star, arguing
from their isotropic distribution on the sky and
lack of observed proper motion that the most
likely distance from the sun was about 100 pc.
The objects had peculiar colors, and 3C 48 showed
light variations of 0.4 mag. In a section added following the
discovery of the redshifts of
3C 273 and 3C 48, they pointed out that the size limit of
0.15
pc implied by the optical light variations was important in the
context of the huge distance and luminosity implied by
taking the redshift to result from the Hubble expansion.

A detailed analysis of 3C 48 and 3C 273 was published
by Greenstein and Schmidt
(1964).
They considered explanations of the redshift involving (1) rapid motion
of objects in or near the Milky Way, (2) gravitational
redshifts, and (3) cosmological redshifts. If 3C 273
had a transverse velocity comparable with the radial
velocity implied by its redshift, the lack of an observed
proper motion implied a distance of at least 10 Mpc
(well beyond the nearest galaxies). The corresponding absolute magnitude
was closer to the luminosity of galaxies than stars.
The four quasars with known velocities were all receding;
and accelerating a massive, luminous
object to an appreciable fraction of the speed of light
seemed difficult. Regarding gravitational redshifts,
Greenstein and Schmidt argued that the widths of the
emission lines required the line emitting gas to be
confined to a small fractional radius around the massive
object producing the redshift. The observed symmetry
of the line profiles seemed unnatural in a gravitational
redshift model. For a 1
M object,
the observed
H flux
implied an electron density
Ne
1019 cm-3, incompatible with the observed
presence of forbidden lines in the spectrum. The emission-line
constraint, together with a requirement that the massive
object not disturb stellar orbits in the Galaxy, required
a mass 109
M. The
stability of such a
"supermassive star" seemed doubtful in the light of theoretical work by
Hoyle and Fowler (1963a),
who had examined such objects
as possible sources for the energy requirements of extragalactic
radio sources. Adopting the cosmological explanation of the
redshift, Greenstein and Schmidt derived radii for
a uniform spherical emission-line region of 11 and 1.2 pc for 3C 48 and
3C 273, respectively. This was based on the
H luminosities
and electron densities estimated from the
H, [O II], and [O
III] line ratios. Invoking light travel time constraints based on
the observed optical variability
(Matthews and Sandage
1963;
Smith and Hoffleit
1963),
they proposed a model
in which a central source of optical continuum was surrounded by
the emission-line region, and a still larger radio emitting region.
They suggested that a central mass of order 109
M might provide
adequate energy for the lifetime of
106 yr
implied by the jet of 3C 273 and the nebulosity of 3C 48.
This mass was about right to confine the line emitting gas,
which would disperse quickly if it expanded at the observed
speeds of 1000 km s-1 or more. Noting that such a mass would
correspond to a Schwarzschild radius of ~ 10-4 pc,
they observed that "It would be important to know whether
continued energy and mass input from such a `collapsed'
region are possible". Finally, they noted that there
could be galaxies around 3C 48 and 3C 273
hidden by the glare of the nucleus. Many features of this
analysis are recognizable in current thinking about AGN.

The third and fourth quasar redshifts were published by
Schmidt and Matthews
(1964),
who found z = 0.425 and 0.545 for
3C 47 and 3C 147, respectively.
Schmidt (1965)
published redshifts for 5 more quasars. For 3C 254, a redshift z = 0.734, based
on several familiar lines, allowed the identification
of C III] 1909 for the
first time. This in turn allowed
the determination of redshifts of 1.029 and 1.037 from
1909 and
2798 in 3C 245 and CTA 102, respectively. (CTA is a radio
source list from the Caltech radio observatory.) For
3C 287, a redshift of 1.055 was found from
1909,
2798, and another
first, C IV 1550.
Finally, a dramatically higher redshift of 2.012 was determined for 3C 9 on the basis of
1550 and the
first detection of the Lyman
line
of hydrogen at 1215.
The redshifts were large enough
that the absolute luminosities depended significantly
on the cosmological model used.

Sandage (1965)
reported the discovery of a large population
of radio quiet objects that otherwise appeared to resemble quasars.
Matthews and Sandage
(1963)
had found that quasars
showed an "ultraviolet excess" when compared with
normal stars on a color-color (U-B, B-V)
diagram. This led to a search technique in which
exposures in U and B were recorded on the same photographic
plate, with a slight positional offset, allowing rapid
identification of objects with strong ultraviolet continua.
Sandage noticed a number of such objects that did not
coincide with known radio sources. These he called "interlopers",
"blue stellar objects" (BSO),
or "quasi-stellar galaxies" (QSG).
1
Sandage found that at magnitudes fainter than 15, the UV excess objects
populated the region occupied by quasars on the color-color
diagram, whereas brighter objects typically
had the colors of main sequence
stars. The number counts of the BSOs as a function of apparent
magnitude also showed a change of slope at ~ 15m,
consistent with an extragalactic population of objects at
large redshift. Spectra showed that many of these objects
indeed had spectra with large redshifts, including
z = 1.241 for BSO 1. Sandage estimated that
the QSGs outnumbered the radio loud quasars by a factor ~ 500,
but this was reduced by later work (e.g.,
Kinman 1965;
Lynds and Villere 1965).

The large redshifts of QSOs immediately made them potential tools
for the study of cosmological questions.
The rough similarity of the emission-line strengths of QSOs to
those observed, or theoretically predicted, for planetary nebulae
suggested that the chemical abundances were
roughly similar to those in our Galaxy
(Sklovskii 1964;
Osterbrock and Parker
1966).
Thus these objects, suspected by many astronomers
to lie in the nuclei of distant galaxies, had reached fairly
"normal" chemical compositions when the Universe was considerably
younger than today.

The cosmological importance of redshifts high enough to make
L visible was quickly
recognized. Hydrogen gas in intergalactic
space would remove light from the quasar's spectrum at
the local cosmological redshift, and continuously distributed
gas would erase a wide band of continuum to the short wavelength
side of the L emission
line (Gunn and Peterson
1965;
Scheuer 1965).
Gunn and Peterson set a tight
upper limit to the amount of neutral hydrogen in intergalactic
space, far less than the amount that would significantly retard the
expansion of the Universe.

All these absorption systems had
zabs < zem. They could be interpreted
as intervening clouds imposing absorption spectra at the appropriate
cosmological redshift, as had been anticipated theoretically
(Bahcall and Salpeter
1965).
Alternatively, they might represent material expelled from the quasar,
whose outflow velocity is subtracted from the cosmological
velocity of the QSO. However, PKS 0119-04 was found to have
zabs > zem, implying material that was in some
sense falling
into the QSO from the near side with a relative velocity of 103
km s-1
(Kinman and Burbidge
1967).
Today, a large fraction
of the narrow absorption lines with zabs substantially
less than zem are believed to result from intervening
material. This includes the so-called "Lyman alpha forest"
of closely spaced, narrow
L lines that punctuate the
continuum to the short wavelength side of the
L emission line,
especially in high redshift QSOs. The study of intervening
galaxies and gas clouds by means of absorption lines in the
spectra of background QSOs is now a major branch of astrophysics.

A different kind of absorption
was discovered in the spectrum of PHL 5200 by
Lynds (1967).
This object showed broad absorption bands on the short wavelength
sides of the L,
N V 1240, and
C IV 1550 emission
lines, with a sharp boundary between the emission and absorption.
Lynds interpreted this in terms of an expanding shell of gas around
the central object. Seen in about 10 percent of radio quiet QSOs
(Weymann et al. 1991),
these broad absorption lines (BALs) are among the many
dramatic but poorly understood aspects of AGN.

The huge luminosity of QSOs, rapid
variability, and implied small size caused some astronomers to
question the cosmological nature of the redshifts.
Terrell (1964)
considered the possibility that the objects were
ejected from the center of our galaxy. Upper limits on the proper
motion of 3C 273, together with a Doppler interpretation of the
redshift, then implied a distance of at least 0.3 Mpc and an age at
least 5 million years.
Arp (1966),
pointing to close pairs of
peculiar galaxies and QSOs on the sky, argued for noncosmological
redshifts that might result from ejection from the peculiar galaxies
at high speeds or an unknown cause.
Setti and Woltjer (1966)
noted that ejection from the Galactic center would imply for the QSO
population an explosion with energy at least 1060 ergs, and
more if ejected from nearby radio galaxies such as Cen A as suggested by
Hoyle and Burbidge
(1966).
Furthermore, Doppler boosting would cause us to see more blueshifts
than redshifts if the objects were ejected from nearby galaxies
(Faulkner, Gunn, and
Peterson 1966).
Further evidence for cosmological redshifts was provided by
Gunn (1971),
who showed that two clusters of galaxies containing QSOs had the same
redshifts as the QSOs. Also,
Kristian (1973)
showed that the
"fuzz" surrounding the quasistellar image of a sample of QSOs was
consistent with the presence of a host galaxy.

1 Here we adopt the now common practice
of using the term "quasi-stellar object" (QSO) to refer to
these objects regardless of radio luminosity
(Burbidge and Burbidge
1967).
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